A common form of pointing device for use with computers and the like is referred to as a “mouse”. The computer user moves the device over a surface to move a cursor on the computer screen. The amount of motion and the direction of motion of the mouse are sensed by the device and determine the distance and direction in which the cursor moves on the screen. Inexpensive mice based on a ball that rolls over the surface have been used for some time. The ball turns two cylinders that sense the distance and direction of motion. Unfortunately, the ball picks up grease and other dirt from the surface and transfers this material to the cylinders. The resulting coating on the cylinders interferes with the motion of the cylinders, and hence, the devices must be cleaned periodically. The cleaning operation is awkward and time consuming.
Mice based on optical sensing avoid this problem. Originally, such mice had to be moved over a special pad that had grid lines that were sensed by the device. The need to use this special pad made these devices less attractive than the mechanical mice discussed above. Recently, optical mice that do not require such pads have been developed. These mice include a light source that illuminates the surface under the mouse at a shallow angle, which accentuates the structural details of the surface. An image sensor in the mouse records an image of the illuminated surface periodically. By comparing two successive images, the displacement of the mouse between the times at which the images were taken can be determined.
Unfortunately, this type of optical mouse does not function properly on a glass-covered surface such as the glass tops used on many desks or other work surfaces. The glass covers are used to protect the underlying surface. The top surface of the glass is too smooth to provide an image that has sufficient structure to measure the displacement of the mouse. While the surface under the mouse often has the required structure, the imaging sensor in the mouse is positioned relative to the light source in a manner that prevents this underlying surface from being utilized. Hence, optical mice have not been useable on many glass covered desktops.